This application claims priority from Australian Application PS2667, filed May 30, 2002.
The present invention relates generally to a fluid turbine. In particular, the invention concerns a wind turbine, although it may also be driven by other fluids such as water.
Machinery for extracting power from the wind has been known for centuries. Early types, known as windmills, were used to power mills for grinding grain and for similar applications. They typically had a rotor with a number of radial blades or fabric sails and means for orienting the rotor to face into the wind when required and for xe2x80x9cfeatheringxe2x80x9d or furling the blades or sails when not required or in strong winds. They were of low sophistication and low efficiency. Many were of considerable size in order to generate useful quantities of mechanical power.
Agricultural and other applications in which operation of mechanical devices such as pumps was required led to development of new and improved machines for harnessing wind power. An example still widely seen is the ordinary xe2x80x9cwindmillxe2x80x9d used for pumping well water for farm stock. Such machines typically have a rotor with a number of simple sheet metal blades thereon, and drive mechanical equipment directly through gearing. They were (and are) typically smaller in rotor diameter and more efficient than the earliest windmills and their rotors operate at somewhat higher speeds. Vane arrangements are provided to face the rotor into the wind and to orient it suitably to avoid damage in high winds.
With the development of electrical technology, machines were developed to convert wind power to electrical power through generators. This trend began with small scale machines, used for remote area power supply, but has now become much more important, with many machines of large scale being used to supply large-area power grid systems alongside coal-fired and other types of power stations. Improving understanding of fluid flow in the 19th and 20th centuries has led to the development of new types of machines and to better design techniques. The term xe2x80x9cwindmillxe2x80x9d is giving way to xe2x80x9cwind turbinexe2x80x9d for most applications as the machines themselves have developed in sophistication and scale.
Although machines have been developed in which the axis of rotation of the rotor is vertical (e.g. the Darrieus and Savonius types well known in the art), the most common wind turbine type has a rotor with a horizontal axis of rotation and a small number of radial blades, so that the rotor appears similar to an aircraft propeller. Such horizontal axis wind turbines have been built in very large sizes and with great sophistication in their design.
However, the use of large numbers of turbines of this type for power generation is still marginal economically and is controversial, due to their size, their often unattractive appearance, their noise and even their interference with radio-frequency transmissions caused by the large rotating blades. Also, their sophistication in itself has the problem that ongoing maintenance can be expensive and difficult, greatly affecting life-cycle costs and even their ability to be used in some places where suitable people to maintain them are not available.
The present invention is intended to address these problems. The wind turbine disclosed herein is comparatively simple, robust, easy and comparatively cheap to manufacture and maintain in the intended sizes. Yet it is believed to have surprisingly good efficiency and so to be able to provide useful amounts of power in the intended size and cost range. Further, it is believed to have satisfactory noise characteristics, and a reasonable appearance. While not necessarily of the theoretically highest possible efficiency, it is believed that the wind turbine of the present invention provides a useful alternative to other types available due to this combination of properties.
The present invention was conceived in an attempt to improve on the wind turbine disclosed by Cobden in U.S. Pat. No. 4,415,306 and Australian Patent No. 563265 (hereinafter referred to as the Cobden wind turbine). As described below, this machine differed radically from conventional horizontal-axis wind turbines of the xe2x80x9chigh speedxe2x80x9d type having two or three radial propeller-style blades often used for electric power generation, and of the xe2x80x9clow speedxe2x80x9d type having a larger number of radial blades and typified by the agricultural windmill used for pumping water. Although quiet and visually acceptable, it was believed that the Cobden turbine""s performance could be improved.
Wind turbines similar to the Cobden turbine were disclosed by Aylor in U.S. Pat. No. 4,781,523 as offering higher efficiency. One embodiment, very similar to the Cobden turbine, had multiple blades arranged peripherally on a rotor with their lengths parallel to the rotation axis of the rotor with fairings provided for causing air to flow radially through the blades. In another embodiment, the rotor had blades extending forwardly and outwardly from a hub, with air flowing outwardly and backwardly through the blades. In both embodiments, particular relationships between air inlet and outlet areas and flow directions were specified, as discussed further below. Neither embodiment was considered particularly cheap or simple to manufacture, due to the required shape of the flow deflector(s) and rotor support body. U.S. Pat. No. 4,684,316 (Karlsson) discloses a somewhat similar arrangement that was felt likely to have high cost in large sizes and high aerodynamic losses from the non rotating parts upstream of the rotor.
Many Wind turbines have been disclosed in which higher efficiency than in conventional types was to be obtained by enclosing a bladed rotor in a duct with a diffuser section downstream of the rotor. These enable faster flow through the blading which can accordingly be more effective in generating power, and reduce blading tip losses. Some examples are in U.S. Pat. Nos. 4,021,135, 4,075,500, 4,132,499, 4,324,985, 4,422,820.
However, Kling, in U.S. Pat. No. 4,147,472, points out that the costs of most ducted arrangements tend to make their economics unattractive, even where performance improvement is obtained. Kling discloses a shrouded rotor of very small size, with a shroud in the form of a ring with an airfoil cross section developing lift forces that act radially inward. This ring is secured to, and rotates with, radial blades of conventional type. The ring is little longer in the flow direction than the blades themselves, and so can be relatively inexpensive. The effect of the shroud is stated to be to develop a toroidal vortex which increases the flow velocity through the blades without the need for a long diffuser downstream of them. The shroud is disclosed as an adjunct to conventional radial blade wind turbine rotors.
According to the invention there is provided a fluid turbine for extraction of power from a moving fluid, including:
a rotor mounted to a support structure for rotation about a horizontal axis, said rotor having a plurality of blades extending forwardly and outwardly from a hub.
a ring fairing secured to said blades at outer ends thereof and rotatable with said blades about said axis, said ring fairing being concentric with said axis of rotation,
wherein said ring fairing has at at least one peripheral location thereon a radial cross-section shaped to develop in operation of said turbine a circulatory flow about said cross section in such a direction as to increase the velocity of fluid flow between said ring fairing and said hub.
It is preferable that at said at least one peripheral location on said ring fairing said ring fairing develops in operation of said turbine an aerodynamic force directed inwardly and rearwardly of said ring fairing.
Preferably, said ring fairing is of uniform cross section around substantially the entire periphery of said ring fairing.
In a particularly preferred embodiment, said ring fairing cross section is so shaped, sized and oriented that said circulatory flow is sufficient to at least partly offset that change in direction of fluid flow relative to a said blade due to the increase of tangential velocity of the blade with increasing radius of said blade.
The said hub may be of substantially conical shape, with a vertex angle in the range 60 degrees to 120 degrees, More preferably, said vertex angle is in the range 80 degrees to 100 degrees. A conical hub has the advantage of ease of manufacture.
The trailing edges of said blades in operation of said turbine preferably sweep out an at least approximately conical surface. The said conical surface preferably intersects an external surface of said hub at approximately 90 degrees when both said surfaces are seen in a cross section in a radial plane that includes said axis of rotation. This best ensures that flow near the inner ends of the blades is substantially transverse to them. However, the said conical surface may intersect an external surface of said hub at an angle in the range of approximately 75 degrees to 90 degrees when both said surfaces are seen in cross section in a radial plane that includes said axis of rotation.
It is considered satisfactory (although not essential) for each said blade to be of substantially constant cross-sectional shape along its length.
Each said blade may be of airfoil-shaped cross-sectional shape. However, each said blade may be formed of a sheet material and have a cross-sectional shape that is arcuate. This can be a satisfactory approximation to an airfoil section, and gives the advantage of ease of construction.
At least when low cost is desirable, each said blade is preferably substantially untwisted along its length.
It is preferred that the ring fairing cross section is at least approximately of airfoil shape. Preferably, said airfoil shape has a camber line that is concave on a side thereof that is opposite to the blades. However, the ring fairing may be formed of sheet material with said ring fairing cross section being of arcuate shape. It is then preferable that said arcuate shape is concave on a side thereof opposite to the blades.
In a preferred embodiment, said ring fairing cross section has a leading edge and a trailing edge and the distance when viewed in a radial plane including said axis of rotation between said leading and trailing edges is less than twice the maximum chord length of each said blade. That is, the ring fairing is of quite small dimension in the fluid flow direction.